The invention relates to microelectromechanical system (MEMS) processes, and more particularly, to microelectromechanical optical (MEMO) display device processes.
A wide variety of optical devices may currently be fabricated using micromachining and microelectronic fabrication techniques.
In some cases, for example, MEMS devices may comprise optical components, specifically referred to as MEMO display devices. One example of a MEMO display device is the interference modulator described in U.S. Pat. No. 5,835,255, which may be fabricated in an array and used in a reflective display wherein each interference modulator serves as a pixel to provide a desired optical response.
FIG. 1A shows a side view of a conventional interference modulator in two states. Referring to FIG. 1, numeral 102 denotes a pixel in an undriven state and numeral 104 denotes a pixel in a driven state. In the driven state, a mirror plate 110 is in direct contact with a substrate 120 such that the interference modulator absorbs incident light and appears black to a viewer 140 through the substrate 120. In the undriven state, an air gap 112 exists between the mirror plate 110 and the substrate 120 such that the interference modulator appears to be a bright color (for example, blue). Additionally, numeral 130 denotes a post for supporting the mirror plate 110.
FIG. 1B shows a plane view of a conventional semi-finished optical MEMS device and FIG. 1C shows a cross-section of a-a′ of FIG. 1B. Referring to FIGS. 1B and 1C, the conventional semi-finished optical MEMS device 150 comprises a plurality of conductive lines 153 disposed on a glass substrate 152 with a dielectric layer 154 overlaid thereon. A plurality of reflective members 158 is supported by a plurality of posts 156 in a sacrificial layer 171 which will be removed thereafter. The conductive lines 153 are perpendicular to the reflective members 158, and the overlapping areas define a plurality of pixel areas 151a, 151b, 151c, 151d, 151e and 151f. Referring to FIG. 1C, the reflective member 158 typically is a stack layer including a metal layer 161 with high reflectivity, such as Al, and a flexible layer 163, such as Ni. However, when patterning the high reflective layer 161 and the flexible layer 163 using a resist layer 159 as a mask, serious under cut 172 occurs because high reflective layer 161 and flexible layer 163 have distinct etching bias and specifically etching rate of Ni is more than 5 times of Al. Therefore, as shown in FIG. 1C, the high reflective layer 161 often remains to affect display quality or cause short. In some serious cases, edge of the flexible layer 163 is separated from edge of the reflective layer 161 by a distance d1 larger than 2 μm.
U.S. Patent Application Publication No. 2002/0015215 to Miles, the entirety of which is hereby incorporated by reference, discloses a method for forming an interference modulator, comprising patterning an aluminum layer to form a mirror plate.
U.S. Patent Application Publication No. 2003/0152872 to Miles, the entirety of which is hereby incorporated by reference, discloses a method for forming an interference modulator, comprising forming a stack layer on a substrate and exposing a photosensitive layer deposited thereon using the stack layer as a photomask.
U.S. Patent Application Publication No. 2004/0027636 to Miles, the entirety of which is hereby incorporated by reference, discloses a method for forming an interference modulator, comprising forming a light-absorbing layer on a portion of a substrate.